Cold Exposure Induces Tissue‐Specific Modulation of the Insulin‐Signalling Pathway in Rattus Norvegicus

Abstract

Cold exposure provides a reproducible model of improved glucose turnover accompanied by reduced steady state and glucose-induced insulin levels. In the present report we performed immunoprecipitation and immunoblot studies to evaluate the initial and intermediate steps of the insulin-signalling pathway in white and brown adipose tissues, liver and skeletal muscle of rats exposed to cold. Basal and glucose-induced insulin secretion were significantly impaired, while glucose clearance rates during a glucose tolerance test and the constant for glucose decay during a 15 min insulin tolerance test were increased, indicating a significantly improved glucose turnover and insulin sensitivity in rats exposed to cold. Evaluation of protein levels and insulin-induced tyrosine (insulin receptor, insulin receptor substrates (IRS)-1 and -2, ERK (extracellular signalrelated kinase)) or serine (Akt; protein kinase B) phosphorylation of proteins of the insulin signalling cascade revealed a tissue-specific pattern of regulation of the molecular events triggered by insulin such that in white adipose tissue and skeletal muscle an impaired molecular response to insulin was detected, while in brown adipose tissue an enhanced response to insulin was evident. In muscle and white and brown adipose tissues, increased 2-deoxy-D-glucose (2-DG) uptake was detected. Thus, during cold exposure there is a tissue-specific regulation of the insulin-signalling pathway, which seems to favour heat-producing brown adipose tissue. Nevertheless, muscle and white adipose tissue are able to take up large amounts of glucose, even in the face of an apparent molecular resistance to insulin.

Cold-induced PGC-1α expression modulates muscle glucose uptake through an insulin receptor/Akt-independent, AMPK-dependent pathway

“…Studies of insulin production and action and of glucose uptake in homeothermic animals exposed to cold revealed that, in this particular physiological condition, glucose is efficiently mobilized, despite low blood insulin levels and an apparent tissue-specific resistance to insulin's action (7,11,41,45). Therefore, it became clear that mechanisms that act independently of insulin signaling through the IR-IR substrates (IRSs)-Akt pathway play an important role in the control of glucose disposal in coldexposed animals.…”

“…Tissue uptake of 2-DG was measured in vivo according to a method described previously (44) with minor modifications (11). The rats were anesthetized and then injected through the portal vein with 6.0 Ci of 2-DG and [ 14 C]sucrose in the presence or absence of 0.1 U of insulin in 0.4 ml of isotonic phosphate buffer (pH 7.4) with 0.1% defatted BSA.…”

“…The supernatant was centrifuged (100,000 g, 60 min, 4°C) to obtain the cytosol fraction and the pellet, which was suspended in STE buffer ϩ 1% NP-40, kept on ice for 20 min, and centrifuged (100,000 g, 20 min) to obtain the membrane fraction. The fractions were treated with Laemmli buffer with 100 mM DTT and heated in a boiling water bath for 5 min, and aliquots (0.2 mg of protein) were subjected to SDS-PAGE and Western blotting with anti-GLUT4 antibodies, as described elsewhere (11).…”

“…Under this condition, animals mobilize glucose more efficiently, despite molecular resistance to insulin signal transduction in skeletal muscle and adipose tissue (11). Because cold exposure significantly induces PGC-1␣ expression in muscle, we decided to evaluate the effect of PGC-1␣ expression inhibition on glucose clearance and insulin signal transduction in type I and type II fiber-rich portions of gastrocnemius muscle from control and cold-exposed Wistar rats.…”

“…The exposure of homeothermic animals to a cold environment has been widely used as a model for studying glucose homeostasis and insulin action (11,45). Under this condition, animals mobilize glucose more efficiently, despite molecular resistance to insulin signal transduction in skeletal muscle and adipose tissue (11).…”